1. Field of the Invention
The present invention relates to a method and an apparatus of controlling an electric vehicle.
2. Description of the Related Art
FIG. 7 shows a schematic of construction of an electric vehicle in commercial use. A chopper 3 connects to a battery 1 by way of a magnet switch 2. A battery voltage is converted from direct current (hereinafter referred to as xe2x80x9cDCxe2x80x9d) to DC by the chopper. Then, the thus converted voltage is supplied to a direct current motor 4 which is subjected to control. The direct current motor 4 has a rotary shaft connecting to a tire 6 by way of a speed reduction gear 5. Also shown in FIG. 7 includes an electromagnetic brake 7.
During travel, the electric vehicle may take the following operations:
1. The magnet switch 2 is turned on.
2. Conductivity (continuity) of the chopper 3 varies in accordance with an angle of an accelerator (not shown in FIG. 7).
3. The voltage applied to the direct current motor 4 varies according to the conductivity of the chopper 3, thus controlling the direct current motor 4 during travel of the electric vehicle.
For achieving the operations described above, the magnet switch 2 keeps turned on during travel of the electric vehicle.
For stopping the electric vehicle, the driver stops operating the accelerator, to thereby reduce the direct current motor 4 in speed. Then, the direct current motor 4 reaching substantially 0 in speed opens an exciting coil of the electromagnetic brake 7, thus generating a braking force (torque) to a brake system for allowing the electric vehicle to make a complete stop. Together with the stop of the electric vehicle, the chopper 3 stops operation.
In other words, during stop of the electric vehicle, opening the exciting coil of the electromagnetic brake 7 allows an exciting current to become 0, allowing the electromagnetic brake 7 to have a spring force for braking operation.
During travel of the electric vehicle, the construction according to the related art in FIG. 7 requires energization of the exciting coil of the electromagnetic brake 7, so as to release the braking force (torque) caused by the spring force. In consideration of fail safe, the above description is required for securing the braking force (torque) for the electromagnetic brake 7, so as to prevent a possible failure of the electromagnetic brake 7 and a control circuit.
Thereby, the electric vehicle having the battery 1 as a driving power source has to continuously keep supplying exciting current to the electromagnetic brake 7 during travel of the electric vehicle. Power consumption attributable to the continuous supply of the exciting current to the electromagnetic brake 7 is about 30% of a motor current consumed when the electric vehicle is making a flat traveling. Considering energy efficiency, the figure 30% is not ignorable (minor).
The direct current motor 4 is preferred as small as possible. For generating a great force (torque) with the direct current motor 4 that is small in size, there is provided the speed reduction gear 5 (gear type). Setting the speed reduction gear 5 is responsible for greater capacity of the battery 1, which is also disadvantageous in terms of efficiency as well as energy saving.
Diminishing the gear of the speed reduction gear 5 in size may damage the teeth of the gear. In this case, the electric vehicle may be inconvenient especially for those who are physically disabled or weak, for the following causes:
* The braking force (torque) is applied to the rotary shaft of the direct current motor 4 in such a manner that a great braking force (torque) can be generated by a small braking force. Therefore, the damage of the teeth of the gear of the speed reduction gear 5 may cause inability in securing the braking force (torque) of the electromagnetic brake 7.
It is an object of the present invention to provide a method and an apparatus of controlling an electric vehicle.
It is another object of the present invention to provide the method and the apparatus of controlling the electric vehicle that is advantageous especially to those who are physically disabled or weak.
According to a first aspect of the present invention, there is provided a method of controlling an electric vehicle which is so constituted as to drive a synchronous motor by way of a switch and an inverter circuit. A battery acts as a source of driving a controlling power. The synchronous motor has a permanent magnet acting as an outer rotor. The method comprises the following operations of:
1) detecting that the electric vehicle has an acceleration signal of zero;
2) detecting that an actual speed of the electric vehicle is less than a predetermined percent of a rated speed;
3) supplying a current to a winding of a fixed phase of the synchronous motor, by way of the inverter circuit; and
4) generating a braking force.
According to a second aspect of the present invention, there is provided an apparatus of controlling an electric vehicle, comprising:
1) a battery acting as a source of driving a controlling power;
2) a synchronous motor connected to the battery by way of a switch, the synchronous motor having a permanent magnet acting as an outer rotor and having a position detector; and
3) an inverter circuit interposed between the synchronous motor and the battery, the inverter circuit including a controller which outputs a control signal for controlling the synchronous motor.
The controller comprises:
a) a brake speed detector receiving a speed signal and an acceleration signal of the electric vehicle, and carrying out a first detection for detecting that the acceleration signal is zero and a second detection for detecting that an actual speed of the electric vehicle is less than a predetermined percent of a rated speed;
b) an optimum current command operator receiving the first detection and the second detection carried out by the brake speed detector, and setting up a current signal for generating a braking torque; and
c) a pulse width modulation signal generator receiving the first detection and the second detection carried out by the brake speed detector, and selecting a switching element of a fixed phase of the inverter circuit.
The other objects and features of the present invention will become understood from the following description with reference to the accompanying drawings.